The present invention relates to a process for reductive plutonium stripping from an organic reprocessing solution into an aqueous, nitric acid solution by use of an electrolytic current.
In particular, the present invention relates to a process for the separation of plutonium out of an organic phase, which is developed in the recovery of irradiated nuclear fuel and/or fertile material. In the recovery process, irradiated nuclear fuel and/or fertile material is dissolved in an aqueous, strong acid to form an aqueous phase or starting solution (feed solution) containing, among other things, uranium ions, and plutonium ions. The uranium and plutonium ions are then transferred from the aqueous phase into an organic reprocessing solution (organic phase) comprised of an organic extraction agent, such as tributyl phosphate (TBP), dissolved in a diluent. This transfer of uranium and plutonium to the organic phase is known as coextraction. The plutonium is present in this organic phase in its 4+ valence state. The organic phase is then treated to selectively strip the plutonium from the organic reprocessing solution to an aqueous solution by reducing the plutonium to its 3+ valence state.
A process for the reductive stripping of plutonium from an organic extraction solution into an aqueous, nitric acid solution by use of an electrolytic current was suggested in German Patentschrift No. 19 05 519 for the separation of the fissionable materials uranium and plutonium which are present in an organic solution which was developed in the recovery of irradiated nuclear fuel- and/or fertile materials. In this process, the fissionable material uranium remained in the organic reprocessing solution, in contrast with the fissionable material plutonium, which was present in the organic solution in its fourth valence states, was reduced to the third valence state and transferred on the basis of the solubility relationship into the aqueous phase. This phase is a diluted aqueous phase which contains nitric acid, and nitrous acid which is also always present in such a diluted nitric acid containing aqueous phase. This nitrous acid is in a position to reoxidize Pu(III), and accordingly hydrazine was used as a stabilization agent for Pu(III) in the aqueous stripping solution. Hydrazine or hydrazine nitrate, respectively, prevents the reoxidation of Pu(III) by reacting with the nitrous acid faster than Pu(III) reacts with the nitrous acid. The reaction rate for the Pu(III)-HNO.sub.2 reaction depends on the acid concentration. The reaction proceeds quickly and autocatalytically at the usual acid concentrations of 0.8 to 1 mol/l HNO.sub.3.
Indeed, based on the knowledge that below a certain nitric acid concentration the nitrous acid no longer has an oxidizing, but rather a reducing effect on the Pu(IV)-Pu(III) pair, which could be deduced from the Potential-pH-Diagrams for the HNO.sub.3 /HNO.sub.2 /NO.sub.x /Pu system (Pourbaix-Diagrams, M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon Press, 1966), it was attempted to introduce nitrous acid or nitric oxide as the reduction agent in the plutonium-uranium-reduction-extraction process (PUREX Process) in the aqueous reductive stripping solution and thus avoid the addition of a Pu(III) stabilization agent. Difficulties, however, arose with this technical process. The redox characteristics of the HNO.sub.2 /Pu system are not only dependent on pH, but also dependent on concentration. On the one hand, one must try to attain a high nitrite concentration, for example 0.01 to 1 mol/l, while on the other hand a low acid concentration down to 0.1 mol/l HNO.sub.3 must be guaranteed in order to attain sufficiently high reduction rates. Moreover, the organic phase has an extracting effect on HNO.sub.2. Finally, additional problems can arise as a result of polymer formation of Pu(IV) in low acid concentrations.
Because of these technical process difficulties, preference is given to electrolytic reduction of plutonium and its stripping from the organic phase into the aqueous phase, in which a stabilization agent for Pu(III), as for example hydrazine, must be contained. Such an aqueous stripping phase can comprise, for example, a diluted nitric acid with about 0.6 mol/l HNO.sub.3, which contains about 0.2 mol/l hydrazine, as disclosed in German Patentschrift No. 19 05 519. Subsequent to this stripping, the stabilization agent must, however again be destroyed, in order to be able to supply the aqueous, plutonium-containing solution for further processing. Moreover, for the continuous production of the aqueous stripping solution, hydrazine must be taken from storage, which presents technical safety problems particularly in nulear plants.